The present invention relates to a bending machine for use in bending pipes, mainly water or oil pipes, at a plurality of positions longitudinally thereof. More particularly, the invention relates to a bending machine comprising means for feeding a length of workpiece as maintained in a predetermined posture to a bending section by advancing the workpiece axially thereof, a plurality of bend dies arranged coaxially with each other and adapted to bend a portion of the workpiece fed to the bending section to different curvatures, a first actuator for axially moving the bend dies to selectively place the bend dies in an operative position, and a second actuator for moving the bend dies together along a plane to selectively place the bend dies in the operative position on the plane.
This type of bending machine, which selects the bend die to be used by moving the bend dies relative to the workpiece feed means, has the advantage over the type of bending machine that selects the bend die to be used by moving the feed relative to the bend dies. The advantage resides in that, while the feed means tends to be large and heavy since the feed means often needs to be longer than the workpiece it supports, the actuator or actuators for selecting the bend die to be used and a support structure therefor can be of small and light design and hence low cost.
The type of bending machine having the above advantage conventionally has the second actuator comprising a hydraulic or pneumatic cylinder.
However, such a conventional bending machine still has room for improvement. The cylinder constituting the second actuator, though operable at slow speed, is very difficult to accurately stop at an intermediate position between stroke ends thereof from the point of view of control technique. This not only renders a bending operation time-consuming and inefficient but results in varying gaps between the workpiece and bend dies. Consequently, a twist of the workpiece and other inconveniences due to the presence of gaps make it difficult or impossible to bend the workpiece so as to follow peripheral surfaces of the bend dies, and hence poor bending precision. Furthermore, since it is very difficult to vary the cylinder stopping position intermediate between the stroke ends for reasons of control technique and cost, it is virtually impossible to replace the bend dies with those having different curvatures. The limitation to the variety of curvatures results in limination to use of the bending machine. Moreover, it is very difficult to operate the cylinder at variable speeds from the point of view of control technique and cost. Therefore, where, for example, in bending a workpiece including a projecting object 1a such as an attaching metal piece as shown in FIG. 8, one of the bend dies is moved to the operative position simultaneously with the introduction to the bending section of the workpiece portion carrying the projecting object in order to shorten the bending time for high working efficiency, the projecting object and the bend die tend to interfere with each other such as by colliding depending on the size and orientation of the projecting object. This inevitably results in an inefficient bending operation in which the workpiece portion carrying the projecting object is first fed into the bending section and then one of the bend dies is moved to the operative position.